The invention relates to transmitting data by using optical links. It relates more particularly to optical switches used in communications networks equipped with this type of link. Such switches serve in particular to perform routing functions in the network.
Generally, optical switches are designed for selectively directing one or more optical signals received from one or more transmission lines to other lines as a function of a command. The switch may be a purely space-division switch. It may also be a time-division switch, i.e. capable of selectively delaying the input signals by durations that vary as a function of the command. Time-division switches are used in particular to resolve contention in packet-switching systems.
Unfortunately, currently-known switches suffer from drawbacks. In particular, they impart interference noise, they have limitations as regards speed of switching, and they impart losses.
An object of the invention is to remedy the above drawbacks. To this end, the invention provides a switching apparatus for selectively transmitting n input optical signals to one or more outlet ports as a function of a direction command, the input signals having the form of modulated optical waves that have different respective wavelengths, the apparatus including:
n passive couplers, each of which is provided with a number of outlets equal to the number k of the outlet ports, and with at least one inlet suitable for receiving one of the input signals; and
k wavelength multiplexers, each of which is provided with an outlet constituting one of the outlet ports, and with n inlets set to respective ones of the wavelengths;
and for each multiplexer, that one of its inlets which is set to one of the wavelengths is coupled via an amplifying optical gate to one of the outlets of that one of the couplers that is organized to receive an input signal carried by the same wavelength.
The invention thus implements a space-division switch in which each input signal can be directed to one or more outlet ports. By means of the presence of amplifying optical gates, i.e. optical amplifiers generally optimized for enabling gain to be varied rapidly as a function of an electrical or an optical command, the power per channel can be maintained at an appropriate level. Furthermore, the fact that switching is performed by on/off control of the gates makes it possible to simplify control. In addition, the use of optical gates makes rapid switching possible, and gives a high isolation ratio, the isolation ratio being defined as the ratio between the output signal power when the gate is conducting and the output signal power when the gate is blocked.
Another major advantage is the absence of interference noise. This is due to the fact that the waves applied to the inputs of the multiplexers have wavelengths that are all different, and that the multiplexers act as filters. For a given channel, each multiplexer eliminates the amplification noise of the other channels. This advantage would not be achieved if mere couplers were used instead of multiplexers. Another advantage of the presence of the multiplexers in the circuit proposed is that the signal-to-noise ratio for each channel is independent of the number of channels directed to the same outlet. Finally losses are reduced to as low as possible, and all these properties enable a high bit-rate to be achieved.
In a variant embodiment, the apparatus includes n wavelength converters suitable for delivering respective ones of the input optical signals as a function of external optical signals having the form of modulated optical waves having arbitrary wavelengths.
In this way, the presence of wavelength converters removes any constraint as regards the wavelengths of the external signals to be directed. Advantageously fixed wavelength converters are used so as to guarantee carrier stability.
The invention also provides a time-division switch enabling any signal applied to one of its inlets to be directed to one or more of its outlets, each outlet delivering an output signal that is delayed relative to the input signal, with a delay that can be varied as a function of the applied command. For this purpose, in the apparatus at least kxe2x88x921 outlets of each coupler are connected to respective ones of the inlets of the multiplexers via optical delay lines dimensioned to create propagation delays that are all different.
Advantageously, each delay line is placed so as to connect one of the outlets of a coupler to one of the optical gates.
Finally, the invention provides a space-and-time-division switch implemented by putting a time-division switch of the invention and a space-division switch of the invention in cascade.